1. Field of the Invention
The present invention relates to a process for forming a fine pattern constituted of a polyhydroxyalkanoate on a substrate. Particularly, the present invention relates to a process for forming a fine pattern constituted of a polyhydroxyalkanoate by an inkjet system.
2. Related Background Art
Fine pattern-forming techniques such as lithography, which have been developed for production of integrated semiconductor circuits, are utilized for production of liquid crystals, electronic parts, and like articles, and further are employed as fine working technique such as MEMS. With the expansion of the application fields thereof, improvement of the technique is demanded for producing the fine patterns more simply at a lower cost.
In recent years, fine patterning techniques employing an inkjet system are attracting attention. The inkjet system has been developed as an ink ejection mechanism of a printer for ejecting an ink onto desired spots on a paper sheet to print letters or to draw a picture. The inkjet system ejects fine liquid droplets onto desired spots with high precision. By the inkjet system, a pattern can be formed in a line width of micron meters by ejecting a patterning material on a substrate.
For example, Japanese Patent Application Laid-Open No. H11-340129 describes formation of a resist pattern by ejecting a solution of a resist material through an inkjet head onto a pattern formation surface.
Japanese Patent Application Laid-Open No. 2002-324966 describes drawing of a circuit pattern on a wiring substrate by ejecting an electroconductive metal paste by an inkjet system.
On the other hand, high polymers capable of constructing various chemical structures are very useful and indispensable in industries and daily lives. Furthermore, the high polymers are promising because of the characteristics such as durability, shielding ability, insulating property, reactivity, and so forth.
For formation of a fine pattern from such a high polymer, the aforementioned inkjet system is investigated. Specifically, the high polymer is firstly dissolved in a solvent to prepare an ejection liquid composition for the inkjet system; this ejection liquid composition is ejected through an inkjet head onto a substrate; and the solvent of the ejected liquid is dried to obtain a pattern of the high polymer. In this method, the organic solvent is indispensable for dissolving the high polymer.
However, the organic solvent generally has a low surface tension to wet the substrate, so that the liquid droplets ejected onto the substrate can flow undesirably on the surface of the substrate to spread the pattern, making difficult the formation of the fine pattern. Further, in ejection by the inkjet system, the ejection accuracy can be decreased. For example, the ejection liquid composition having a surface tension of lower than 3 mN/m (=30 dyn/cm) can wet more the periphery of the nozzle opening to cause deviation of the trajectory of the liquid droplets.
To prevent the undesirable spreading of the pattern on the substrate or decrease of the ejection accuracy, the concentration of the high polymer in the ejection liquid composition may be increased to increase the viscosity of the ejection liquid composition. However, the increase of viscosity may prevent smooth ejection of the liquid composition through the ejection nozzle of the ink head, or may prevent sharp cutting of the ejection of the liquid composition at the ejection nozzle, or may cause threading of the ejected liquid between deposited dots on the substrate. For example, an ejection liquid composition having a viscosity higher than 20 cp is not ejected smoothly to cause frequently clogging of the nozzle, or does not form stable meniscus at the nozzle tip to make difficult the control of ejection quantity or ejection timing.
Furthermore, the organic solvent has a high volatility disadvantageously. For example, the organic solvent of the composition can evaporate to deposit the polymer around the ejection nozzle and to change the quantity or direction of the ejection to deviate the liquid deposition position or to cause clogging of the nozzle.
In use of the organic solvent, the ink head should be resistant to the solvent, and a ventilation apparatus should be equipped for evaporation of the organic solvent, which causes increase the cost.
To offset the above disadvantages, Japanese Patent Application Laid-Open No. 2003-7459 discloses an aqueous solution as the ejection liquid by employing a water-soluble medium compound of 20 or less molecules as the constituting material, but it does not mention a polymer of a higher molecular weight.